1. Field of the Invention
The present invention relates to cation exchange resins, and more particularly to cation exchange resins which are capable of trapping alkali metal ions from organic solvents and suited for use in organic solvent purifying apparatus or the like, and also to the use of the resins.
2. Description of the Prior Art
Inorganic absorbents such as activated carbon, silica gel, activated alumina and synthetic zeolite have long been used in the chemical industry, food industry, petroleum industry, etc. for purifying gases or liquids and for trapping traces of components. Especially with the use of diversified production processes of increased complexities, development of various new processes and innovations in production techniques in recent years, sophisticated methods of purifying materials are required for manufacturing products of improved qualities, so that the adsorption purification technique is resorted to in various fields. The adsorption purification method is based on the principle of trapping molecules of gases such as air and radioactive waste gases, or of liquids such as water or other liquids in fine pores of the inorganic adsorbent of specified size. However, the inorganic adsorbents heretofore used originally contain large amounts of alkali metals and are therefore limited in use because of the difficulty encountered in avoiding contamination with alkali metal ions although having high ability to adsorb water gases.
Porous polymers have been developed in recent years as adsorbents free of the drawback of these inorganic adsorbents. Typical of such organic porous polymers are polystyrene resins prepared by suspension (emulsion) polymerization and easily controllable in particle size and pore size. These resins are widely used for liquid chromatography utilizing their characteristics.
On the other hand, the purification method of adsorbing ions with an ion exchange resin employs an insoluble porous film of synthetic resin serving as a kind of adsorbent and having an ion group with ion exchange ability. When this method is used for cation exchange, the synthetic resin employed is a high polymer acid having an acidic group introduced therein and removes cations from aqueous solutions. Examples of useful synthetic resins are typically ion exchange celluloses and chemically coupled porous polymers. The ion exchange celluloses comprise a cellulose resin having an ion exchange group, e.g. --CH.sub.2 CH.sub.2 N.sup.+ (C.sub.2 H.sub.5).sub.2, --CH.sub.2 CH.sub.2 N.sup.+ (C.sub.2 H.sub.5).sub.3, --CH.sub.2 COO.sup.-, --CH.sub.2 CH.sub.2 SO.sub.3.sup.- or --PO(OH).sup.- O, attached thereto through ether linkage and thereby given an ion exchange capacity.
Although the aforementioned porous polymer is merely a copolymer of styrene, divinylbenzene or the like and is nonpolar, the chemically coupled porous polymers include those comprising styrene having sulfonic acid or like acidic group introduced therein and thereby made polar (extra issue of Chemical Industry, Plant Operation Series, "Adsorption," pp. 143-163(1971), published by Kagaku Kogyo Co., Ltd.), and acrylic ester, acrylamide and like polymers having intermediate polarity imparted thereto. Such chemically coupled porous polymers having medium to strong polarity are known to act on the substances to be removed, selectively in response to delicate differences in polarity, and are generally used in the field of fine chemicals for isolating metabolites, enzymes, proteins, etc., in the field of petrochemicals for example for separating aliphatic hydrocarbons from aromatic hydrocarbons, in the field of waste water treatment for efficiently removing phenol, chlorophenol, alkylbenzenesulfonates, etc. and in other fields.
Nevertheless, the aforementioned porous polymer is substantially easily soluble in organic solvents and has the drawback of being usable for water and dilute aqueous solutions only.
Of the ion exchange resins, the ion exchange cellulose has the problem of being soluble in organic solvents due to the presence of its ion exchange group although the skeletal cellulose resin thereof is generally sparingly soluble in usual solvents such as water, alcohols and ethers.
The chemically coupled porous polymer similarly has the problem of being usable only in aqueous solutions because of low resistance to organic solvents.
Thus, the adsorbents described are all low in resistance to organic solvents, are therefore difficult to use for removing impurities from organic solvents by adsorption and are not usable for purifying organic solvents.
The present invention, which has been accomplished in view of the foregoing situation, provides an adsorbent capable of stably adsorbing metal ions, especially alkali metal ions, from various organic solvents by ion exchange without dissolving or swelling therein, and also a method of purifying organic solvents with use of the adsorbent.
From the above viewpoint, we have directed attention to polyimide resins which are chemically stable and carried out intensive research thereon.
In recent years, polyimide resins have attracted attention as resins having very high chemical stability. Especially, aromatic polyimides are thought most promising because of their high heat resistance. The polymide has a tough structure obtained, for example, by subjecting an aromatic tetrabasic carboxylic acid dianhydride and a primary diamine to a condensation cyclization reaction as represented by the following formula. ##STR2##
There is a tendency to use polyimide resins of higher condensation degree and higher imidation degree which have improved resistance to heat, chemicals, ozone and radiation. Especially, polyimide resins of high condensation degree are obtained by heating a polyamidocarboxylic acid, precursor thereof, at a temperature of at least 300.degree. C. for a long period of time to subject the carboxylic acid groups and the amido groups to condensation cyclization substantially completely.
In view of the fact that the condensation degree is controllable by varying the heating temperature or heating time, we conducted investigations based on the idea that a polyimide resin obtained at a reduced degree of condensation and having polyamidocarboxylic acid (polyamic acid) units partially remaining therein will be useful as an ion exchange material because of their carboxyl groups. Consequently, we have found that such a polyimide resin irreversibly adsorbs and removes mobile cations from organic solvents through ion exchange with good stability, and further that the resin, like usual ion exchange resins, can be readily regenerated with dilute hydrochloric acid or dilute sulfuric acid.
Although conventional polyimide resins include those subjected incompletely to condensation cyclization, for example, for the adjustment of viscosity in forming coatings, these resins are used only for simple structural materials as heat-resistant insulating varnishes or the like, and nothing whatever is known as to the use of such resins as ion exchange materials or adsorbents.